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Actinolite
Actinolite crystal in matrix from Finland
General
Category	Mineral
Chemical formula	Ca2(Mg,Fe)5Si8O22(OH) 2[1]
Identification
Color	Pale to dark green, yellowish green and black.[2] White or grey when in asbestos form
Crystal habit	bladed, fibrous, radial
Crystal system	Monoclinic[2]
Twinning	simple or lammelar
Cleavage	Perfect in two directions. [2]
Fracture	uneven[2]
Mohs scale hardness	5 - 6[2]
Luster	vitreous to dull[2]
Streak	white
Diaphaneity	translucent to transparent
Specific gravity	3.00 (+.10, -.05)[2]
Polish luster	vitreous [2]
Optical properties	Double refractive with anomalous aggregate reaction, biaxial negative.[2]
Refractive index	1.606 - 1.641 (+.014)[2]
Birefringence	.022 - .027[2]
Pleochroism	moderate, yellow to dark green (in stones that are transparent)[2]
Ultraviolet fluorescence	inert[2]
Absorption spectra	faint line at 503nm[2]

Actinolite is an amphibole silicate mineral with the chemical formula Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂.

Mineralogy

Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca₂Mg₅Si₈O ₂₂(OH)₂, and iron-rich ferro-actinolite, Ca₂Fe₅Si₈O ₂₂(OH)₂. Mg and Fe ions can be freely exchanged in the crystal structure. Like tremolite, asbestiform actinolite is regulated as asbestos.

Occurrence

Actinolite is commonly found in metamorphic rocks, such as contact aureoles surrounding cooled intrusive igneous rocks. It also occurs as a product of metamorphism of magnesium-rich limestones.

The old mineral name uralite is at times applied to an alteration product of primary pyroxene by a mixture composed largely of actinolite. The metamorphosed gabbro or diabase rock bodies, referred to as epidiorite, contain a considerable amount of this uralitic alteration.

Some forms of asbestos are formed from fibrous actinolite, the fibres being so small that they can enter the lungs and damage the alveoli.

Gemology

Some forms of actinolite are used as gemstones. One is nephrite, one of the two types of jade (the other being jadeite, a variety of pyroxene).^[2]

Another gem variety is the chatoyant form known as cat's-eye actinolite. This stone is translucent to opaque, and green to yellowish green color. This variety has had the misnomer jade cat's-eye.^[2] Transparent actinolite is rare and is faceted for gem collectors.^[2] Major sources for these forms of actinolite are Taiwan and Canada.^[2] Other sources are Madagascar, Tanzania, and the US.^[2]

References

1. ^ http://rruff.geo.arizona.edu/doclib/hom/actinolite.pdf Mineral Handbook
2. ^ ^{a b c d e f g h i j k l m n o p q r s} (Gia), Gemological. Gem Reference Guide. City: Gemological Institute of America (GIA), 1988. ISBN 0-87311-019-6

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
• Mindat.org
• Webmineral.org
• Mindat - uralite